Fishermen plying on the eastern coast of India have helped scientists discover a fresh water ‘river' that forms in the Bay of Bengal just after monsoon season1.
Arrows showing origin of the 'river in sea' n the Bay of Bengal all the way to the end. Locations from where fishermen collected water samples are named along the coast. Credit: Gopalakrishna, V. V. et al.
The ‘river in the sea’ forms in northern Bay of Bengal at the end of the monsoon and ‘vanishes’ gradually after a while. About 100 kilometres wide, it flows southward hugging the eastern coast of India and reaching the southern tip after two and a half months. The seasonal river in the sea was discovered by salinity measurements of sea water samples collected by fishermen along the coast.
The Bay of Bengal receives intense rainfall during the monsoon. This, and the run-offs from the rivers — Ganges, Brahmaputra, Mahanadi, Godavari and Krishna — bring around 1100 cubic kilometres of freshwater into the bay between July and September.
"This very intense freshwater flux into a relatively small and semi enclosed basin results in dilution of the salt in seawater," says one of the lead researchers V. V. Gopalakrishna, a scientist at the National Institute of Oceanography (NIO), Goa. The diluting effect gets concentrated in the upper 40 metres of the bay waters, resulting in a stark contrast between surface freshwater and saltier water below, he says.
The presence of low salinity water (called stratification in oceanography parlance) over the Bay of Bengal prevents vertical mixing of sea water. This results in the accumulation of more heat in the near-surface layers, Gopalakrishna says. The sea surface temperature remains above 28.5°C, a necessary condition to maintain deep atmospheric convection and rainfall. Similarly, strong salinity stratification close to the coast would mean more intense tropical cyclones, he says.
Earlier studies have shown that salinity plays a crucial role in influencing climate variability and cyclone activity. However, lack of in-situ observations in the bay hampered clarity on the temporal and spatial distribution of salinity near the coast.
To fill this gap, the NIO engaged fishermen at eight specific stations along the east coast to collect seawater samples every five days in a clean bottles. The bottles, marked with the date of collection, have been routinely brought back to NIO since 2005 for salinity measurements.
The new dataset revealed a salinity drop of more than 10 grams per kilogram of water in the northern Bay of Bengal at the end of the summer monsoon. This relatively fresh water propagates southward as a narrow (100 km wide) strip along the eastern coast of India, according to the scientists.
"Local fishermen have been a great help in developing this coastal network," Matthieu Lengaigne of the collaborating institute, French Institut de Recherche pour le Développement, told Nature India.
This knowledge may help us validate models used to predict cyclones evolution in the Bay of Bengal," Lengaigne says. The study demonstrates the possibility of building a scientifically usable observational network at low cost by relying on local communities, he says.
Following the success of the Indian programme, Sri Lankan oceanographers have initiated a similar network around Trincomalee and Colombo.
While the scientists have so far focused on salinity measurements, they contend that the coastal seawater sampling programme could also be used for regular monitoring of other oceanic parameters such as phytoplankton or bacteria.
